Hey!
Whenever you declare a UFUNCTION, you have to define the FunctionName_Validate() and FunctionName_Implementation() instead of FunctionName() in the.cpp (where the validate one has bool as return type (for your needs you’ll want to let it return true all the time) and the implementation void)
You should probably take a look at https://wiki.unrealengine.com/Networking/Replication!
Good luck 